The present invention relates to a composition for treating lithographic printing plates and more particularly it relates to an etching liquid for rendering non-image areas oil-unreceptive and dampening liquid. The normal lithographic printing is accomplished by applying both water and an ink to the surface of printing plates so that the image areas selectively accept a coloring ink and the non-image areas selectively accept water and then transfering the ink on the image areas to a substrate such as paper. Therefore, in order to obtain excellent prints it is necessary that the difference between oleophilicity of image areas and hydrophilicity of non-image areas is sufficiently great and when water and an ink are applied to the surface of printing plates the image areas accept sufficient amount of ink and the non-image areas accept no ink. For this purpose, various methods for rendering the plates oil-unreceptive have been proposed depending on the kind of the plate materials and printing processes. That is, hitherto, as the lithographic printing plates there have been used presensitized plates (PS plates) having a support of metals such as aluminum, etc.; printing plates obtained by using electrophotography (e.g., electrofax); master papers which comprises a substrate of paper on which a pigment is coated together with a binder; plates produced by photographical method which uses silver salt diffusion method as disclosed in Japanese Patent Publication No. 30562/73, etc. Various treating liquids for oil-unreceptive the non-image areas of such printing plates are sold, but many of these treating liquids are usable for only specific printing plates and no etching liquids and dampening liquids satisfactorily and equally usable for various printing plates have been known.
Some of proposals have been made as such kind of treating liquids, but none of them satisfy overall conditions in lithographic printing. In general, treating liquids used in offset printing are required to have the following characteristics.
(1) They can render the non-image areas sufficiently oil-unreceptive so that no ink stick to the non-image areas.
(2) They do not damage ink-receptivity of image areas.
(3) They do not accelerate emulsification of ink.
(4) They have no corrosive action on printing 20 machines and metal portions of printing plates.
(5) They are stable against heat and light for a long time.
(6) They do not rot.
(7) They do not deteriorate printability of printing plates.
(8) They can be used effectively regardless of kind of printing plates, kind of inks and kind of printing substrate (e.g., paper).
The inventors already proposed such treating liquids as satisfy the above requirements in Japanese Patent Publications Nos. 29723/73, 15762/76, 15763/76 and 29441/76 and Japanese Patent Laid-Open Application No. 36402/74. However, most of the etching liquid or dampening liquids proposed hitherto including those proposed by the inventors are effective only for the specific printing plates as mentioned above and besides they are mostly improved in the above requirements (1), (2), (3), (5) and (7) among those (1)-(9) and very few of them take into account the corrosion action of the requirement (4), namely, rust preventing property and the rot of the requirement (6), namely, the mold growing in the treating liquids. The etching liquids or dampening liquids now proposed or practically used have more or less these defects. For example, Japanese Patent Publication No. 5123/72 discloses a treating liquid which comprises (a) an alkali metal polyphosphate, (b) phosphoric acid or citric acid and (c) a sodium or potassium nitrate, perchlorate, permanganate or persulfate, for the purpose of rust prevention of printing plates using a support of aluminum, zinc or a stainless steel. However, this treating liquid has the rust preventing effect on only specific metals and has no sufficient inhibiting effect against rot.
Lithographic printing plates are made from various materials such as iron, aluminum, aluminum alloys, copper, other metals and alloys thereof, plastics, rubbers, etc. Especially, container for dampening liquid and container for etching liquid which is recently used in completely automatic operation of from plate making to plate treating and printing are made of various metals and so they inevitably undergo corrosion. Furthermore, the metallic portions of printing machines are always under the conditions of being corroded, e.g., corrosions of the portions of the printing machines due to scatter of treating liquids or inclusion of etching liquid into container for dampening liquid during printing. Besides, even the printing plates per se which have a metallic support are also corroded.